I. Field of the Invention
Recently, "pay-TV" systems have become popular in the U.S.A. In such pay-TV systems, viewers who have contracts with the broadcasting station (subscribers) can correctly receive a program, while viewers who do not have contracts with the broadcasting station (nonsubscribers) cannot. In order to prevent correct reception by nonsubscribers, scrambling methods which process a video signal are used, so that a nonsubscriber who receives a program cannot watch a correct picture. Various scrambling methods for processing a video signal have been proposed. A pay-TV system using such a scrambling method provides a subscriber with a decoder for decoding (or descrambling) the scrambled signal to obtain the original signal.
II. Description of the Prior Art
One conventional scrambling method inverts is white and black levels about a predetermined reference level or turn-up level. For example, a video signal S.sub.v shown in FIG. 1A has its polarity inverted about the turn-up level V.sub.m which is set between the white and black levels to form a scrambled video signal S.sub.vs, shown in FIG. 1B. In this case, if vertical and horizontal synchronizing pulses of the video signal were inverted, it would be difficult to descramble the signal. Thus, only a video period is inverted.
In practice, when the level-inversion scrambling method is used in a broadcasting system, a scrambled pattern is formed in an arbitrary arrangement of level-inverted and non-inverted field periods, as shown in FIG. 2 and is transmitted repeatedly. Simultaneously, an identification signal ID which indicates that the broadcasting program is scrambled, and a key code signal KY which is used for descrambling the scrambled pattern, are inserted at predetermined positions ID+KY of FIG. 2 of a vertical blanking interval, respectively. In one scrambling method, for example, the identification signal ID having a constant level is inserted in the sixteenth horizontal scanning period of the vertical blanking interval, and the key code signal KY is inserted in the seventeenth and eighteenth horizontal scanning periods of the vertical blanking interval.
To receive the scrambled television signal as described above, the level-inverted field period is detected based on the key code signal KY, and then descrambling is performed by reinverting the signal during this period to obtain the original video signal. The turn-up level for reinversion is formed at the receiver corresponding to the turn-up level V.sub.m at the transmitter. If the turn-up level at the receiver was subjected to the influence of variations in the electronic components having ambient temperature fluctuations, for example, the reinversion would not result in the reproduction of the original waveform. As shown in FIG. 3A, when the signal S.sub.vs which is inverted about the turn-up level V.sub.m at the transmitter is reinverted about the identical level V.sub.m at the receiver, the waveform equal to the original one of the signal S.sub.v shown in FIG. 1A can be obtained. However, as shown in FIG. 3B, when the signal is reinverted about a turn-up level having an offset of an amount .DELTA.V.sub.m, the signal S.sub.v ' having the distorted waveform indicated by the solid line will be obtained instead of the correct signal S.sub.v indicated by the dotted line.
In order to solve this problem, it has been suggested that the level of the identification signal ID set at the identification level related to the turn-up level V.sub.m be transmitted. As shown in FIG. 4, the identification signal ID which has the level V.sub.m ' and is inserted in the sixteenth horizontal scanning period (16H) has a known value relative to the turn-up level V.sub.m (in FIG. 3, V.sub.m '=V.sub.m). Therefore, the identification signal ID is transmitted for serving also as a reference level signal for descrambling.
Even if the level of the video signal varies during transmission, the turn-up level based on the level of the signal V.sub.m ' and used for reinversion provides accurate descrambling because the level of the identification signal ID varies with the video signal. Thus, no special adjustment need be performed at the receiver side.
There has also been suggested a system in which a scrambled broadcasting program is transmitted after midnight, for example, and is first recorded on a tape by a VTR, and the recorded tape is later reproduced for descrambling. In this system, the scrambled signal is recorded directly, and when being reproduced, the reproduction signal is descrambled through a decoder in order to obtain a correct image. However, when the scrambled signal which has a turn-up level (to be referred to as an inverse level hereinafter) V.sub.m is recorded or reproduced, the following problem occurs due to nonlinear characteristics of the VTR and variations in their other characteristics.
FIG. 5 shows the input/output characteristics of the VTR. A curve A indicates signal characteristics of the normal portion (FIG. 2) of the signal, and a curve B indicates the signal characteristics of the signal portion inverted with respect to a 50% inverse level (half-way between the black level and the white level). As is apparent from FIG. 5, the signal characteristics of the normal field are different from those of the inverted field. The distortion in the reinverted signal becomes different from that of the normal portion of the signal. The reproduced pictures flicker at the time that the normal portion is inverted or the inverted portion is reinverted when the scrambled signal S.sub.vs in FIG. 2 is descrambled.
A turn-up level control circuit must be used in the receiver system described above. A viewer or a person who installs a descrambler must adjust the descrambler. When the person who installs the descrambler performs the adjustment of the descrambler, the installation cost is increased. In addition to this disadvantage, when the viewer replaces a VTR with a new one, readjustment of the descrambler is required. When the viewer adjusts the descrambler, the adjustment requires skill. As a result, fine adjustment cannot be performed, and the viewer watches the screen pictures with degraded quality.